Writing a stream of 9 bit values as bytes to a file in C

Question

I have an array with integer values from 0-511 (9 bits max). I am trying to write this to a file with fwrite.

For Example, with the array:

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Answer 1

You need a bit buffer.

Since you are writing 8 bits at the time, you must have data type that can hold at least 9+7 bits at minimum. uint16_t would do, but I recommend using size that would at least as big as your native int. Make sure you use unsigned types to avoid shifting issues.

uint32_t bitBuffer = 0;  // Our temporary bit storage
uint32_t count = 0;      // Number of bits in buffer

Let's assume we have single data:

uint32_t data9b = 257;  // 1 0000 0001

Adding bits to buffer is simple; just shift bits at the end of the buffer, and combine with OR.

bitBuffer |= (data9b << count); // At first iteration, shift does nothing
count += 9;                     // Update counter

After 9 bits are added, we can flush 8 bits to file.

while(count >= 8) {
    writeToFile(bitBuffer & 0xFF);  // Mask out lowest bits with AND
    bitBuffer >>= 8;                // Remove written bits
    count -= 8;                     // Fix counter
}

After each cycle you have 0 - 7 bits left over in the buffer. At the end of all data, if you finish with non-multiple of 8 bits, just write remaining contents of bitBuffer to file.

if(count > 0)
    writeToFile(bitBuffer);

Answer 2

Ok, so did it using bit shifting, oring (can also do with *, '+', % and /) but shift is more appropriate / readable, imo.

// Your data, n is the number of 9-bits values
uint16_t dat[] = { 257, 258, 259 };
int i,n = sizeof(dat)/sizeof(*dat);

// out file
FILE *fp = fopen("f8.bin","w");

uint16_t one = 0;
int shift = 0;
uint8_t b;

// main loop
for(i=0 ; i<n ; i++) {
    b = (dat[i] << shift) | one; // one is remainder from previous value
    one = dat[i]>>(8-shift);     // Move the remaining MSb to the right
    shift = (shift+9) % 8;       // next shift
    fwrite(&b, 1, 1, fp);        // write our b byte
}
// remainder, always have a remainder
fwrite(&one, 1, 1, fp);
fclose(fp);

Had fun :-)